1. Field of the Invention
The invention, in general, relates to a lightweight photovoltaic system consisting of a plurality of photovoltaic modules arranged in rows and columns and, more particularly, to a system of the kind referred to including a plurality of photovoltaic modules consisting of photovoltaic panels disposed over a self-supporting flat and lightweight boards of water-repellant material for mounting at a predetermined distance from a substructure. Preferably, the self-supporting boards of the system engage each other in a gapless arrangement and are held together by guide and connecting fixtures including a frame and a wire tension system.
2. The Prior Art
Among renewable energy sources photovoltaic systems can be applied in many ways not least because of their modular construction from individual photovoltaic modules. Nowadays, their main field of application relates to consumer applications, i.e., they are used to transform solar energy into electric energy. For this purpose, the photovoltaic system has to be mounted on substructures accessible to sun light. As a rule, such substructures are exposed surfaces of roofs or side walls of buildings. In accordance with conventional engineering standards, such as, for instance, the German Industrial Norm (DIN), photovoltaic modules are mounted on flat roofs at an inclination of up to 5°, and they must be capable of withstanding wind load induced forces as specified in DIN 1055-T4 and DIN EN 1991-1, Part 1-4. Suction wind loads have a significant effect upon the dimensions of a photovoltaic system mounted on a flat roof. For instance, in wind load zone II of the Federal Republic of Germany, wind loads are based upon values between 0.82 kN/m2 and 1.02 kN/m2. Thus, any wind load calculations are to be based upon a value of ˜1.00 kN/m2. For Europe as a whole higher values must be assumed to apply. For Europe, load values may be assumed in general to correspond to those of German wind load zone III.
In general, the effective suction wind load is safely absorbed and overcome by the weight of the photovoltaic system preventing it from being lifted off. A predetermined safety margin based upon changes in the frictional forces assumed to exist between the substructure, e.g., the cover or surface of the roof, and the gravity system, is also factored in. Accordingly, a force of about 1.00 kN/m2 (Federal Republic of Germany, suction wind loading zone II, height of building >20 m) must be introduced into the roof to prevent the lift off of an unsecured photovoltaic system. This assumed surface load is usually converted into a distributed load and is diverted into the surface of the roof. Therefore, significant point or distributed loads are generated in the area of force transmission between the supports of the photovoltaic system and the cover of the roof cover which can significantly damage the cover as well as the insulation of the roof. During the winter months this load would be increased further by the weight of accumulated snow and ice.
A different problem may result from the photovoltaic systems being usually mounted in a horizontal disposition, forming a wind tunnel between the lower surface of the system and the surface of the substructure which causes additional lifting forces to be introduced into the system. In order to prevent an increase in the overall load of the photovoltaic system, it has, therefore, been found necessary to surround such systems with a wind deflector. Yet another problem arises in connection with inherent load reserves of the substructure which usually is the roof structure. While it may generally be assumed that the structural arrangement of heavy roof structures provides for a load reserve, the supporting framework of light roofs may be assumed to have scarcely any load reserve. Hence, a system depending on gravity cannot be employed on a light roof structure.
Most commercial systems place an emphasis upon weight. The weight is usually absorbed by large support surfaces of stone blocks. Another possibility is to build plastic tubs subsequently filled with ballast such as, for instance, gravel. Yet another system relies upon large surface pedestals placed upon a support surface. As in other systems, the pedestals or feet are weighed down by heavy stone plates, stone blocks or gravel. An alternative would be a lightweight photovoltaic system of the kind provided by the instant invention.
German patent specification DE 101 02 918 A1 discloses photovoltaically effective laminated panels for generating solar electricity and solar heat. The panels are rigidly and adhesively connected to each other and disposed between a support plate and a weatherproof colorless or highly transparent glass plate. German utility model DE 202 15 867 U1 discloses a system for securing photovoltaic modules on point foundations or anchoring points between which wire cables are drawn for connecting a plurality of retaining means. The retaining means are rigidly connected to the photovoltaic modules by support elements attached to the wire cables. U.S. Pat. No. 6,148,570 A discloses a photovoltaic system in which individual photovoltaic modules are connected to each other by I-shaped retaining members which are rigidly connected to the cover of a roof. Surrounding wind deflectors are intended to prevent wind from lifting the system off the roof.
German patent specification DE 100 37 757 A1 discloses a covering element with integrated photovoltaic modules for use as an external cover of a roof or exterior wall of a building. The individual covering elements are rigidly connected to each other by groove and feather connections and pins. German patent specification DE 100 50 624 A1 discloses a light structural panel made of a double wall of textile material and expanded polystyrene (EPS).
U.S. Pat. No. 5,746,839 A to Dinwoodie et al. and assigned to PowerLight Corporation of Berkeley, Calif., USA relates to a lightweight photovoltaic system in which each photo voltaic module consists of a lightweight board of water repellant material with a photovoltaic panel mounted at a predetermined distance. By means of a marginal profile, the lightweight boards are held together in a predetermined arrangement similar to a groove and feather connection. While the entire photovoltaic system is surrounded by a wind deflector structured as a closed frame, the individual photovoltaic modules are without any frame. The shape of the frame system is maintained by a tensioned wire system which also serves to press the individual photovoltaic modules against each other. The tensioned wire system is made up of several tension wires drawn between the rows and columns of the photovoltaic modules and attached to opposite internal surfaces of the frame system. A gap is provided between the lightweight panels and the substructure which may be a flat roof, for instance. The gap serves to equalize the pressure above and below the lightweight photovoltaic system. Together with the wind-deflecting surrounding frame system the photovoltaic system is intended to withstand any occurring suction wind loads without additional safety measures, its lightness notwithstanding.
Evidently the structural measures mentioned were found to be insufficient, for US 2003/0164187 A1, a later application in the name of the assignee's referred to supra and deemed to be the prior art most closely related to the instant invention, discloses a lightweight photovoltaic system differing from the one of the predecessor '839 patent by through-bores in every one of the photovoltaic modules for improved equalization of pressure between the upper and lower surfaces thereof. However, the through-bores reduce the pressure resistance of the lightweight boards. Moreover, flow-conducting radial barriers are arranged in the intermediate space between the photovoltaic panel and the lightweight board. The photovoltaic panels have to be positioned at a relatively large distance from each other so that air may flow between them into the through-bores. However, this reduces the effective photovoltaic surface. Finally, two lightweight boards may at their upper surfaces be connected by additional connecting elements. These relatively complex measures taken in addition to those proposed by U.S. Pat. No. 5,746,839 clearly indicate that securing a lightweight structural system on a substructure without directly engaging it to prevent lift off under suction wind loads is rather problematic indeed.